Open Data supplied by Natural Environment Research Council (NERC)

Niskin Bottle

The Niskin bottle is a device used by oceanographers to collect subsurface seawater samples. It is a plastic bottle with caps and rubber seals at each end and is deployed with the caps held open, allowing free-flushing of the bottle as it moves through the water column.

Standard Niskin

The standard version of the bottle includes a plastic-coated metal spring or elastic cord running through the interior of the bottle that joins the two caps, and the caps are held open against the spring by plastic lanyards. When the bottle reaches the desired depth the lanyards are released by a pressure-actuated switch, command signal or messenger weight and the caps are forced shut and sealed, trapping the seawater sample.

Lever Action Niskin

The Lever Action Niskin Bottle differs from the standard version, in that the caps are held open during deployment by externally mounted stainless steel springs rather than an internal spring or cord. Lever Action Niskins are recommended for applications where a completely clear sample chamber is critical or for use in deep cold water.

Clean Sampling

A modified version of the standard Niskin bottle has been developed for clean sampling. This is teflon-coated and uses a latex cord to close the caps rather than a metal spring. The clean version of the Levered Action Niskin bottle is also teflon-coated and uses epoxy covered springs in place of the stainless steel springs. These bottles are specifically designed to minimise metal contamination when sampling trace metals.

Deployment

Bottles may be deployed singly clamped to a wire or in groups of up to 48 on a rosette. Standard bottles have a capacity between 1.7 and 30 L, while Lever Action bottles have a capacity between 1.7 and 12 L. Reversing thermometers may be attached to a spring-loaded disk that rotates through 180° on bottle closure.

Originator's Protocol for Data Acquisition and Analysis

Discrete nutrient samples were collected from a variety of depths spanning the full depth of each CTD cast. Water was collected from depth using a stainless steel CTD frame fitted with a Sea-Bird SBE 32 twenty-four way carousel (s/n 32-45661-0621) equipped with 24, twenty litre OTE external spring water samplers. However, due to the complete loss of the CTD package at station 12, water was subsequently collected using a stainless steel CTD frame fitted with a Sea-Bird SBE 32 twenty-four way carousel (s/n 32-19817-0243) equipped with 20, ten litre OTE external spring water samplers (rosette positions 1-20) and 4, twenty litre OTE external spring water samplers (rosette positions 21-24). Nutrient concentrations (silicate, phosphate and nitrate) were analysed using a segmented-flow nutrient autoanalyser with colourimetric detection (Skalar SANplus SYSTEM analyser (type 4000) fitted with a Skalar autosampler (type 1000)). More detailed information on the sampling and analysis of nutrient samples can be found from pages 44-50 of the cruise report .

Limits of detection

The limits of detection and error between duplicate samples were determined by the originators. The limits of detection were defined as twice the level of background noise averaged out over the course of the cruise. The background noise levels in digital units (the arbitrary unit used by the Skalar autoanalyser software) were measured at the start and at the end of 10 of the 72 analyses. These were then averaged to give an averaged background noise level for the whole cruise. This number was then multiplied by 2. The concentration per digital unit was taken from the calculations used to determine the blank nutrient level in the artificial seawater matrix. Multiplying this number by the concentration per unit gives the limit of detection (Table 1).

Table 1. The limits of detection for the chemistries

Chemistry

Limit of detection (µmol l -1 )

Nitrate + nitrite

0.15

Silicate

0.39

Phosphate

0.04

The errors for the nutrient analysis were worked out using a duplicate set of samples run during each analysis. A drift sample was included in each analysis to indicate if the baseline moves over the course of the run. These drift samples are included in pairs, i.e. 2 drift samples from the same batch are run one after the other. The first pair of these drifts was used as the duplicate samples in each of the 72 analyses. The error for the each analysis was determined using the equation below. These errors were then averaged over the cruise to obtain the error for the whole cruise (Table 2).

100 * (A - B) * C, C = 2/(A + B)

Where A is the largest of the duplicates and B is the smallest.

Table 2. The errors associated with each of the chemistries

Chemistry

Error in data (%)

Nitrate + nitrite

0.69

Silicate

0.36

Phosphate

0.45

BODC Data Processing Procedures

Data arrived at BODC in one Microsoft Excel format file which contained all the CTD casts sampled for nutrients during the cruise. The data received were loaded into the BODC database using established BODC data banking procedures. Absent data values were removed and data were screened in-house prior to loading. Data were loaded into BODC's database without any further changes.

Please note:the supplied parameters may not have been sampled from all the bottle firings described in the table above. Cross-match the Sample Reference Number above against the SAMPRFNM value in the data file to identify the relevant metadata.